Recently more and more used are active matrix driving systems with good display qualities such as contrast, display capacity and response time. Among the systems, a liquid crystalline display device of thin film transistor (TFT) system is dominating for a television, a view finder and a personal computer and so on. Also a STN system display device is largely employed for a display device of a personal computer and others, because of its low production cost owing to its relatively simple structure and its large display capacity compared with the active matrix system display device.
Recent developmental tendency in this field is focused on downsizing, mobilizing, saving of consuming power and responding with high speed in the liquid crystalline display device. Thus required are such liquid crystalline compounds and liquid crystalline compositions having low driving voltage (i.e. low threshold voltage) and low viscosity.
The threshold voltage (Vth) is a function of dielectric anisotropy (.DELTA..epsilon.) as represented by the equation below (Mol. Cryst. Liq. Cryst., 12, 57 (1970): EQU Vth=.pi.(K/.epsilon..sub.0.DELTA..epsilon.).sup.1/2
where K is a elastic constant and .epsilon..sub.0 is a dielectric constant under vacuum.
As to be understood from the equation, two ways to lower Vth can be considered either increasing .DELTA..epsilon. or reducing K. However, as the control of K is difficult in the state of art technology, it is required to use liquid crystalline materials having large .DELTA..epsilon., which has stimulated vigorous development of liquid crystalline compounds having large .DELTA..epsilon..
While viscosity is a factor controlling the response speed of liquid crystalline molecule to the electric field, it is desirable to use liquid crystalline compounds having low viscosity as the majority to prepare the liquid crystal composition with high response speed.
Shown below are typical compounds (7) and (8) in Laid-open Japanese Patent Publication No. Hei 2-233 626, developed as liquid crystalline materials for low driving voltage which is usable for the liquid crystal display device with TFT system. ##STR3##
Both compounds (7) and (8) have a 3,4,5-trifluorophenyl group on one terminal of the molecule and hence are expected to be liquid crystalline materials for low driving voltage. However, toward the request of further reduction of driving voltage, the compound (7) (.DELTA..epsilon.=10) is hardly satisfy the request because of its small dielectric anisotropy. On the other hand, the compound (8) (.DELTA. .epsilon.=15) is unsuited to high speed responding, because an ester group existing in the center of the molecule make viscosity high although it is possible to achieve low voltage driving due to its large dielectric anisotropy. Thus such a compound that satisfies both low consumption power and high speed response is not yet known.
It is also known that a difluoromethyleneoxy group acts to increase .DELTA..epsilon. of the compound, which is similar to the ester group mentioned above. ##STR4##
Both compound (9) (DE-19 531 165 Al) and compound (10) (WO 96/11 897) are liquid crystalline compounds having a difluoromethyleneoxy group, and these two patent specifications disclose that these compounds are useful for components of liquid crystalline composition which are capable of low voltage driving and high speed response due to their large .DELTA..epsilon. and relatively low viscosity.
As to the production method of these difluorobenzyl ether, the following producing method is disclosed in the patents DE-19 531 165 Al and Laid-open Japanese Patent Publication No. Hei 5-255 165: ##STR5##
where X' represents fluorine or chlorine; L.sub.3 and L.sub.4 each independently represents hydrogen or fluorine; Hal represents halogen; R' represents alkyl, and ##STR6##
where Ph represents-phenyl.
The production route shown in Scheme 3 has multiple steps including alkylation after the etherification between the intermediate (A) and the phenol derivative. Then, the formation of by-products is anticipated.
As to the compound (A) used as a starting material in the method shown in Scheme 3, A. Haas et al. has reported the method of production (Scheme 5; Chem. Ber., 121, 1329-1340 (1988)): ##STR7##
where Y' represents hydrogen, chlorine, nitro, trifluoromethyl or t-butyl.
The production route shown in Scheme 5 is a method of fluorinating the carbonyl of the benzaldehyde derivative (a) by sulfur tetrafluoride followed by brominating via photoreaction. Sulfur tetrafluoride used for the fluorination is highly reactive gas and a special reaction apparatus is necessary for its use due to its high toxicity. Further, as the bromination of benzyl position is carried out by the photo-reaction (a radical reaction), it is anticipated to generate by-products being brominated at benzyl position on the substituent Y' when Y' is linear alkyl or 1,4-cyclohexylene.
The production route shown in Scheme 4 cannot also be a simple way, because the intermediate thion-O-ester itself requires long synthetic route and the thiocarbonyl must be fluorinated.
As above mentioned, it is not yet known on the method of producing the difluorobenzyl ether derivative which-are useful as liquid crystalline materials, and on the simple and efficient method of producing the benzene derivative which is important for the intermediate. It is anticipated that the demand for the simple production method of the difluorobenzyl ether derivative will increase with their development as liquid crystalline materials.